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Mid-term- The Excitable Cell, Sensory Physiology.docx

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Physiology 1021

Physiology 1021 1 The Excitable Cell Homeostasis and Body Fluid Composition Physiology: the study of function in living organisms, exploring the mechanisms by which the organisms control their internal environments, and attempts to explain the physical and chemical factors responsible for both normal function and pathology External environment: continuous with the outside world o Air coming into the lungs has crossed no cellular membrane to get in o Contents of G.I. tract is part of external until it has been absorbed Internal environment: things must cross cells (absorbed, diffused) o Trillions of cells, each its own entity Homeostasis: the maintenance of relatively stable conditions in the internal environment. Homeostatic mechanisms are dynamic, self-regulatory mechanisms involving all of the organs and tissues of the body. Uses two control systems (both use negative feedback): o Endocrine system: slow, long-acting o Nervous system: fast, short period Negative Feedback Control Systems: operate to maintain homeostasis. Measures a value and compares to set point value. If above or below, initiates a series of responses to get back to set point Extracellular fluid: fluid found outside of the cell (such as plasma) Intracellular fluid: found inside the cell o For the average person, 60% is water o For a 70kg person, the total body water is 42L (28L intracellular fluid, 14L extracellular fluid, 11L interstitial fluid, 3L plasma) Chemical composition of the body fluids *KNOW PATTERNS NOT NUMBERS* Substance Plasma (mOsm/L H2O) Interstitial Intracellular Sodium ions (+) 148 150 15 Potassium (+) 4.8 5.0 150 Calcium (2+) 2.5 2.4 <1 Chloride (-) 102 125 9 Proteins (-) 1.2 0.2 4 In a typical cell, concentration is a salty banana salt, NaCl on the outside, K+ on the inside The compositions of the interstitial fluid and plasma (ecf) are approximately the same o However, intracellular and extracellular fluid are very different o This is possible because the intracellular fluid is separated from the interstitial fluid by a cell membrane which is selectively permeable and contains transport mechanisms Cell membrane structure: selectively permeable membrane, made up of phospholipids arranged into a selectively permeable bilayer o The fatty acid chains of the phospholipid are hydrophobic (and are therefore the major permeability barrier to water and water-soluble substances like ions, glucose etc., but is permeable to fat-soluble substances such as oxygen, alcohol and steroid hormones) o Phospholipid: polar phosphate head, 2 fatty acid chains (lipid, nonpolar) o In water, the O atom exerts a greater pull on the electron, so is negative. Because of this, water attracts to phosphate head, making phosphate hydrophilic and lipid hydrophobic Cell Membrane and Membrane Transport Functions of membrane proteins 1. Receptors (e.g. for hormones, 3. Ion channels (pores) neurotransmitters) 4. Membrane carriers (e.g. for 2. Enzymes transport of glucose) Physiology 1021 2 Membrane transport 1. Endocytosis/exocytosis (pinocytosis for small molecules) 2. Diffusion through lipid bilayer (fat-soluble molecules) 3. Diffusion through protein channels (water-soluble molecules) 4. Facilitate diffusion 5. Active transport Simple diffusion: the movement of molecules from an area of high concentration to low concentration (down the concentration gradient) due to the molecules random thermal motion o Net movement of the substance from A to B o Will continue until the concentration of the substances are the same (now equilibrium) o Equilibrium, no NET movement, molecules are still constantly in movement, but as a whole, they move from A to B Substances must be lipid-soluble (nonpolar) in order to penetrate the lipid region of the lipid bilayer. Lipid soluble molecules: oxygen, carbon dioxide, fatty acids, steroids Factors influencing rate of diffusion for non-polar molecules: 1. Concentration gradient (larger the gradient, faster the movement as a whole) 2. Temperature: (higher temperature promotes faster diffusion) 3. Surface area available for diffusion (larger area means faster diffusion) 4. Size of the molecule (smaller means faster) 5. Viscosity (stickiness of the medium, lower means faster) Ions and water-soluble molecules (including water) cannot diffuse directly through the cell membrane due to the hydrophobic lipid region. These require a pore channel (driving force is the concentration gradient) Factors influencing diffusion in polar molecules: 1. Concentration gradient 2. Size of the molecule 3. Charge (+/-) 4. Number of pores in the membrane (depends on cell, function, tissue) Protein-lined pore: pore is very small, inside has small negative charges (net negative). Pore is somewhat selective, being negative (Cl- wouldnt through, but Na+ would) Facilitated diffusion: attachment of a molecule to an integral protein will cause a conformational change in that protein, moving the molecule across the membrane (driving force is the concentration gradient) o Example glucose: glucose is too big to simply diffuse, so glucose binds to the carrier protein, causing a conformational change. The carrier flips, now facing intracellular fluid. Glucose is then released into the cell Competitive inhibition: similar structure, such as maltose, can bind to the carrier, but will not cause a conformational change as glucose did (blocking all the carriers, glucose cant enter cell) Active transport: a form of carrier-mediated transport chemically specific, can be competitively inhibited. However, moves substances against the concentration gradient, and so requires energy in the form of ATP Na+/K+ Pump: K+ and Na+ passively diffuse through pores, active transport takes input of ATP and pumps K+ back into the cell, and Na+ out (counteracting the diffusion occurring) o 1 ATP 3 Na+ out, bring 2 K+ in o Pump maintains concentration gradient Osmosis and TonicityPhysiology 1021 3 Osmosis: the net movement of water down its concentration gradient (water concentration determined by the number of solute particles in solution, not their size) o Requires a semi-permeable membrane which allows water across, but not the solute o Water moves from dilute solution to concentrated one (more water less water) Osmosis across the cell membrane is affected by: 1. The permeability of the membrane to the solutes in the intracellular and interstitial fluids 2. The concentration gradients of the solutes in the intracellular and interstitial fluids 3. The pressure gradient across the cell membrane Solution: whats being dissolved Solvent: whats doing the dissolving Solution: solute + solvent (in bio, solvent is water) As a general rule, water will move into the area with a higher solute concentration Osmotic pressure: the pressure which would have to be applied to the NaCl solution in order to stop the net influx of water into that solution o Depends on the number (rather than size) of particle in solution Osmole: concentration of a solute expressed in number of particles Osmolality: the number of osmoles per kg of water Osmolarity: the number of osmoles per litre of solution (used interchangeably with osmolality) Tonicity: the ability of a solution to cause osmosis across biological cell membranes o Isotonic: same, no net movement, no osmosis o Hypotonic: lower osmolarity outside, water enters cell (goes from dilute to concentrated, cell swells) o Hypertonic: higher osmolarity outside, water exit
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